Heavy demands are placed on the image quality of x-ray recordings in x-ray image technology. For this type of recording, in particular as performed in medical x-ray diagnostics, an object to be examined is irradiated by x-ray radiation from a virtually punctiform x-ray source. The attenuation distribution of the x-ray radiation on the side of the object opposite the x-ray source is captured in two dimensions. The x-ray radiation attenuated by the object can also be captured line by line, for example in computed tomography systems.
Flat-panel detectors are increasingly used as x-ray detectors in addition to x-ray films and gas detectors, and generally have a matrix-shaped arrangement of opto-electronic semiconductor components as photoelectric receivers. Each pixel of the x-ray recording should ideally correspond to the attenuation of the x-ray radiation through the object on a straight-line axis from the punctiform x-ray source to the location on the detector surface corresponding to the pixel. X-rays that hit the x-ray detector in a straight line from the punctiform x-ray source on this axis are known as primary rays.
The x-ray radiation emitted from the x-ray source is however scattered in the object because of unavoidable interactions, so that scattered rays also hit the detector in addition to the primary rays. These scattered rays, which as a function of properties of the object can cause more than 90% of the entire signal modulation of an x-ray detector in diagnostic images, represent a noise source and make fine differences in contrast harder to identify.
Hence to reduce the proportion of scattered radiation hitting the detectors, what are known as scattered radiation grids are inserted between the object and the detector. Scattered radiation grids consist of regularly arranged structures that absorb x-ray radiation, between which through-channels or through-slots are formed to enable the primary radiation to pass through with as little attenuation as possible. These through-channels or through-slots are aligned toward the focus in the case of focused scattered radiation grids in accordance with the distance from the punctiform x-ray source, i.e. the distance from the focus of the x-ray tube. In the case of unfocused scattered radiation grids the through-channels or through-slots are aligned across the whole surface of the scattered radiation grid vertically to the surface thereof. However, this results in a marked loss of primary radiation at the edges of the image recording, as a larger proportion of the incident primary radiation hits the absorbent regions of the scattered radiation grid at these points.
To achieve an optimal image quality very high demands are placed on the properties of x-ray scattered radiation grids. The scattered rays should on the one hand be absorbed as much as possible, while on the other hand as high a proportion as possible of primary radiation should pass through the scattered radiation grid unattenuated. A diminution of the proportion of scattered radiation hitting the detector surface can be achieved inter alia using a large ratio of the height of the scattered radiation grid to the thickness or the diameter of the through-channels or through slots, i.e. using a high grid ratio, also known as aspect ratio.
There are various techniques and corresponding embodiments for producing scattered radiation grids for x-ray radiation. Thus for example publication DE 102 41 424 A1 describes various production methods and embodiments of scattered radiation grids. For example, lamellar scattered radiation grids are known which are made up of strips of lead and paper. The lead strips serve to absorb the secondary radiation, while the paper strips disposed between the lead strips form the through-slots for the primary radiation. Alternatively aluminum can also be used instead of paper, thereby reducing the costs of the production process. The paper grid uses paper with a low attenuation as a slit or window. The aluminum grid uses aluminum as a slit or window, which has a significantly higher attenuation compared to paper.
With the production of scattered radiation grids with lead films in the form of tapes, experience has shown that this frequently results in material-specific defects, which cause a high error rate or require particularly complicated measures to reduce the error rate. Defects at the cut tape edges are known, since the lead films are very soft and sensitive. Furthermore, tapes may become torn off due to the low mechanical solidity. Faults (cavities, holes) occur in the tape material, particularly when the band thickness is <25 μm. All these defects immediately cause the x-ray image or image artifacts to deteriorate, since the scattered radiation grid is positioned directly in the radiation path (between the patient and image recording system).
In order to protect the sensitive edges of the lead tape from a mechanical load (e.g. when unwinding or winding during the lining with carrier paper), the thin lead tape is inserted into a width which is almost twice as large as is subsequently required in the scattered radiation grid. The lead tape is only cut to the required width immediately before placing the tapes into the grid frames. The overhang is a decrease in production and must be disposed of.
Lead tape is lined with paper in order to produce the scattered radiation grid. The paper serves on the one hand as a carrier for the lead tape and on the other hand has the function of bringing the lead tapes to the required distance so that x-rays can pass almost unhindered between the lead tapes. What is known as the aspect ratio results from the width of the lead tapes, the tape planes of which are aligned in the direction of the focal point of the x-ray tubes, and the thickness of the paper layers, including adhesive thickness, disposed therebetween. The aspect ratio refers to the ratio of gap width to height in the grid.